The expression of plant resistance in response to attack by phytopathogenic organisms is triggered by the plant's ability to specifically recognize a given pathogen. The overall goal of this research project is to gain a better understanding of the plant-pathogen recognition and signal transduction events that lead to the expression of disease resistance in plants. A combination of genetic, molecular and biochemical approaches will be used to identify and characterize plant genes that control disease resistance in Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae. The A. thaliana RPS2 disease resistance gene, which governs recognition of specific P. syringae strains, has been previously isolated and sequenced. However, the role of this gene in pathogen recognition remains unclear. The primary objectives of the experiments described in this proposal are to I) further characterize RPS2 and it's function and 2) identify and characterize new plant loci that are involved in pathogen recognition and the expression of resistance. Three complementary approaches will be used to identify and characterize new loci involved in resistance. The first approach is to identify cellular components that interact with the RPS2 gene product. The genes encoding these components will be identified and their role in resistance studied using reverse genetic techniques. The second approach involves the genetic mapping and molecular cloning of a new locus involved in disease resistance that is defined by the partially susceptible rps132 mutant. The third approach is to identify new genes involved in resistance by isolating and characterizing suppressors and enhancers of well-defined resistance mutants. It is anticipated that these strategies will lead to the identification of loci encoding components of the signal transduction mechanism coupling pathogen recognition to the expression of disease resistance. The identification and characterization of these components will contribute to our understanding of the molecular basis of disease resistance in plants.